CSC 317

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CSC 317

• Chapter 7 Input/Output and Storage Systems

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Chapter 7 Objectives

• Understand how I/O systems work, including I/O
  methods and architectures.
• Become familiar with storage media, and the
  differences in their respective formats.
• Understand how RAID improves disk performance and
  reliability, and which RAID systems are most
  useful today.
• Be familiar with emerging data storage
  technologies and the barriers that remain to be
  overcome.

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7.1 Introduction

• A CPU and memory have little use if there is no
  way to input data to or output information from
  them.
• We interact with CPU and memory only through I/O
  devices connected to them.
• A wide variety of devices (peripherals) can be
  connected to a computer system.
• With various methods of operations
• At different speeds, using different formats and
  data transfer units
• All slower than CPU and internal memory

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7.3 Amdahls Law

• The overall performance of a system is a result
  of the interaction of all of its components.
• Gene Amdahl recognized this interrelationship
  with a formula known now as Amdahls Law.
• This law states that the overall speedup of a
  computer system depends on both
• The speedup in a particular component.
• How much that component is used by the system.

where S is the overall speedup f is the
fraction of work performed by a faster component
and k is the speedup of the faster component.
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7.3 Amdahls Law

• Amdahls Law gives us a handy way to estimate the
  performance improvement we can expect when we
  upgrade a system component.
• On a large system, suppose we can upgrade a CPU
  to make it 50 faster for 10,000 or upgrade its
  disk drives for 7,000 to make them 250 faster.
• Processes spend 70 of their time running in the
  CPU and 30 of their time waiting for disk
  service.
• An upgrade of which component would offer the
  greater benefit for the lesser cost?

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7.3 Amdahls Law

• The processor option offers a 130 speedup
• And the disk drive option gives a 122 speedup
• Each 1 of improvement for the processor costs
  333, and for the disk a 1 improvement costs
  318.
• Disk upgrade seems a better choice.
• Other factors may influence your final decision.

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7.4 I/O Architectures

• We define input/output as a subsystem of
  components that moves coded data between external
  devices and a host system.
• I/O subsystems include
• Blocks of main memory that are devoted to I/O
  functions.
• Buses that move data into and out of the system.
• Control modules in the host and in peripheral
  devices
• Interfaces to external components such as
  keyboards and disks.
• Cabling or communications links between the host
  system and its peripherals.

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7.4 I/O Architectures

• This is a model
  I/O configuration.
• An I/O module
• moves data
• between main
• memory and
• a device interface.

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7.4 I/O Architectures

• I/O can be controlled in four general ways.
• Programmed I/O reserves a register for each I/O
  device. Each register is continually polled to
  detect data arrival.
• Interrupt-Driven I/O allows the CPU to do other
  things until I/O is requested.
• Direct Memory Access (DMA) offloads I/O
  processing to a special-purpose chip that takes
  care of the details.
• Channel I/O uses dedicated I/O processors.

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7.4 I/O Architectures

• Programmed I/O (also called polled I/O)
• CPU has direct control over I/O by sensing
  status, issuing R/W commands, transferring data.
• Operation
• CPU requests I/O by issuing address and command.
• CPU waits until the I/O is completed before it
  can perform other tasks.
• I/O module performs I/O and sets status bits
• CPU checks status bits periodically
• Each device is given a unique identifier
• Simple to implement but wastes CPU processing
• Better suited for embedded- or special-purpose
  systems

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7.4 I/O Architectures

• Interrupt driven I/O
• Solution to CPU waiting, no polling by CPU.
• Device tells the CPU when data transfer has
  completed.
• Basic read operation
• CPU issues read command and do other tasks.
• I/O module receives command from CPU and gets
  data from device and sends an interrupt to the
  CPU.
• Interrupt may be handle by an interrupt
  controller.
• CPU checks for interrupt at the end of
  instruction cycle.
• I/O module must be identified by having either,
• one interrupt request line per I/O module, or
• or all I/O modules may share a single interrupt
  request line (daisy chain).

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7.4 I/O Architectures

• This is an idealized I/O subsystem that uses
  interrupts.
• Each device connects its interrupt line to the
  interrupt controller.

The controller signals the CPU when any of the
interrupt lines are asserted.
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7.4 I/O Architectures

• A system that uses interrupts, the status of the
  interrupt signal is checked at the end of the
  instruction cycle.
• The particular code that is executed whenever an
  interrupt occurs is determined by a set of
  addresses called interrupt vectors that are
  stored in low memory.
• The system state is saved before the interrupt
  service routine is executed and is restored
  afterward.
• In case of simultaneous interrupts,
• Each I/O module has a predetermined priority, or
• Order of I/O modules in the daisy chain
  determines priority.

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7.4 I/O Architectures

• Direct Memory Access (DMA)
• Programmed- and Interrupt driven I/O require
  active CPU participation (CPU is tied up with
  data transfer).
• DMA is the solution for large volume of data
  transfer.
• DMA allows an I/O module to transfer data
  directly to/from memory without CPU
  participation.
• DMA takes the CPU out of I/O tasks except for
  initialization and for actions taken during
  transfer failure.
• CPU sets up the DMA by supplying
• The operation to perform on the device,
• The number and location of the bytes to be
  transferred,
• The destination device or memory address.
• Communication through special registers on the
  CPU.

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7.4 I/O Architectures
DMA configuration

• Notice that the DMA and the CPU share the bus.
• Only one of them can have control of the bus at a
  given time.
• The DMA runs at a higher priority and steals
  memory cycles from the CPU.
• Data is usually sent in blocks.

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7.4 I/O Architectures

• Very large systems employ channel I/O.
• Channel I/O consists of one or more I/O
  processors (IOPs) that control various channel
  paths.
• Slower devices such as terminals and printers are
  combined (multiplexed) into a single faster
  channel.
• On IBM mainframes, multiplexed channels are
  called multiplexor channels, the faster ones are
  called selector channels.
• IOPs are small CPUs optimized for I/O
• They can execute programs with arithmetic and
  branching instructions.

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7.4 I/O Architectures

• Channel I/O is distinguished from DMA by the
  intelligence of the IOPs.
• The IOP negotiates protocols, issues device
  commands, translates storage coding to memory
  coding, and can transfer entire files or groups
  of files independent of the host CPU.
• The host only creates the program instructions
  for the I/O operation and tell the IOP where to
  find them.
• After an IOP completes a task, it interrupts the
  CPU.
• IOP also steals memory cycles from the CPU.

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7.4 I/O Architectures

• This is a channel I/O configuration.

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7.4 I/O Architectures

• Character I/O devices process one byte (or
  character) at a time.
• Examples include modems, keyboards, and mice.
• Keyboards are usually connected through an
  interrupt-driven I/O system.
• Block I/O devices handle bytes in groups.
• Most mass storage devices (disk and tape) are
  block I/O devices.
• Block I/O systems are most efficiently connected
  through DMA or channel I/O.

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7.4 I/O Architectures

• I/O buses, unlike memory buses, operate
  asynchronously.
• Requests for bus access must be arbitrated among
  the devices involved using some handshaking
  protocol.
• This protocol consists of a series of steps.
• Sender and receiver must agree before they can
  proceed with the next step.
• Implemented with a set of control lines.
• Bus control lines activate the devices when they
  are needed, raise signals when errors have
  occurred, and reset devices when necessary.
• The number of data lines is the width of the bus.

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7.4 I/O Architectures

• This is a generic DMA configuration showing how
  the DMA circuit connects to an I/O bus.

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7.4 I/O Architectures

• This is how a bus connects to a disk drive.

Real I/O buses typically have more control lines
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7.4 I/O Architectures

• Example of steps for a write operation to a disk
• DMA places address of the disk controller on the
  address lines.
• Then, DMA raises (asserts) the Request and Write
  signals.
• Disk drive recognizes address. If the disk is
  available, the disk controller asserts a signal
  on the Ready line.
• No other device may use the bus.
• DMA places the data on the data lines and lower
  the Request signal.
• Disk controller sees the Request signal drop, it
  transfers the data from the data lines to its
  buffer, then it lowers its Ready signal.

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7.4 I/O Architectures

• Timing diagrams, such as this one, define bus
  operation in detail.

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7.4 I/O Architectures

• Peripheral Component Interconnect (PCI) bus
• Popular high speed and flexible I/O bus.
• Released by Intel in the 1990's for Pentium
  systems.
• Direct access to memory using a bridge to the
  memory bus.
• Current standard 64 data lines at 66MHz
• Maximum transfer rate is 528MB/sec.
• PCI bus has 49 mandatory signal lines.
• PCI replaced the Industry Standard Architecture
  (ISA) bus.
• Extended ISA (EISA) was available later with a
  higher transfer rate.
• PCI bus multiplexes data and address lines.

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7.5 Data Transmission Modes

• Bytes can be conveyed from one point to another
  by sending their encoding signals simultaneously
  using parallel data transmission or by sending
  them one bit at a time in serial data
  transmission.

• Parallel data transmission for a printer
  resembles the signal protocol of a memory bus
  (nStrobe line is for synchronization

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7.5 Data Transmission Modes

• In parallel data transmission, the interface
  requires one conductor for each bit.
• Parallel cables are fatter than serial cables.
• Compared with parallel data interfaces, serial
  communications interfaces
• Require fewer conductors.
• Are less susceptible to attenuation.
• Can transmit data farther and faster.

Serial communications interfaces are suitable for
time-sensitive (isochronous) data such as voice
and video.
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7.6 Magnetic Disk Technology

• Magnetic disks offer large amounts of durable
  storage that can be accessed quickly.
• Metal or glass disk coated with a magnetizable
  material.
• Disk drives are called direct access storage
  devices, because blocks of data can be accessed
  according to their location on the disk.
• Going to vicinity plus sequential search.
• Access time is variable.
• Magnetic disk organization is shown on the
  following slide.

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7.6 Magnetic Disk Technology

• Disk tracks are numbered from the outside edge,
  starting with zero.

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7.6 Magnetic Disk Technology

• Hard disk platters are mounted on spindles.
• Read/write heads are mounted on a comb that
  swings radially to read the disk.
• Current disk drives are sealed.

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7.6 Magnetic Disk Technology

• The rotating disk forms a logical cylinder
  beneath the read/write heads.
• Data blocks are addressed by their cylinder,
  surface, and sector.
• Disks have same number of bytes per track.
• Variable density and constant angular velocity.
• Tracks and sectors are individually addressable.
• Control information on each track indicates
  starting sector.
• Gaps exists between tracks and sectors.

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7.6 Magnetic Disk Technology

• There are a number of electromechanical
  properties of hard disk drives that determine how
  fast its data can be accessed.
• Seek time is the time that it takes for a disk
  arm to move into position over the desired
  cylinder.
• Rotational delay is the time that it takes for
  the desired sector to move into position beneath
  the read/write head.
• Seek time rotational delay access time.
• Latency is the amount of time it takes for the
  desired sector to move beneath the R/W head after
  seek.

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7.6 Magnetic Disk Technology

• Transfer rate gives us the rate at which data can
  be read from the disk.
• Average latency is a function of the rotational
  speed
• Mean Time To Failure (MTTF) is a
  statistically-determined value often calculated
  experimentally.
• It usually doesnt tell us much about the actual
  expected life of the disk. Design life is usually
  more realistic.

Figure 7.11 in the text shows a sample disk
specification.
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7.6 Magnetic Disk Technology

• Floppy (flexible) disks are organized in the same
  way as hard disks, with concentric tracks that
  are divided into sectors.
• Physical and logical limitations restrict
  floppies to much lower densities than hard disks.
• A major logical limitation of the DOS/Windows
  floppy diskette is the organization of its file
  allocation table (FAT).
• The FAT gives the status of each sector on the
  disk Free, in use, damaged, reserved, etc.

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7.6 Magnetic Disk Technology

• On a standard 1.44MB floppy, the FAT is limited
  to nine 512-byte sectors (There are two copies of
  the FAT).
• There are 18 sectors per track and 80 tracks on
  each surface of a floppy, for a total of 2880
  sectors on the disk. So each FAT entry needs at
  least 12 bits (211 2048 lt 2880 lt 212 4096).
• The disk root directory associates logical file
  names with physical disk locations (FAT entries).
• It occupies 14 sectors starting at sector 19.
• Each directory entry occupies 32 bytes, storing a
  file name and file's first FAT entry.

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7.7 Optical Disks

• Optical disks provide large storage capacities
  very inexpensively.
• They come in a number of varieties including
  Compact Disk ROM (CD-ROM), Digital Versatile Disk
  (DVD), and Write Once Read Many (WORM).
• Many large computer installations produce
  document output on optical disk rather than on
  paper.
• This idea is called COLD-- Computer Output Laser
  Disk.
• It is estimated that optical disks can endure for
  a hundred years. Other media are good for only a
  decade-- at best.

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7.7 Optical Disks

• CD-ROMs were designed by the music industry in
  the 1980s, and later adapted to data.
• This history is reflected by the fact that data
  is recorded in a single spiral track, starting
  from the center of the disk and spanning outward.
• Binary ones and zeros are delineated by bumps in
  the polycarbonate disk substrate. The transitions
  between pits and lands define binary ones.
• If you could unravel a full CD-ROM track, it
  would be nearly five miles long!

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7.7 Optical Disks

• The logical data format for a CD-ROM is much more
  complex than that of a magnetic disk. (See the
  text for details.)
• Different formats are provided for data and
  music.
• Two levels of error correction are provided for
  the data format.
• Because of this, a CD holds at most 650MB of
  data, but can contain as much as 742MB of music.
• CDs can be mass produced and are removable.
• However, they are read only, with longer access
  time that a magnetic disk.

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7.7 Optical Disks

• DVDs can be thought of as quad-density CDs.
• Varieties include single sided, single layer,
  single sided double layer, double sided double
  layer, and double sided double layer.
• Where a CD-ROM can hold at most 650MB of data,
  DVDs can hold as much as 17GB.
• One of the reasons for this is that DVD employs a
  laser that has a shorter wavelength than the CDs
  laser.
• This allows pits and land to be closer together
  and the spiral track to be wound tighter.

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7.7 Optical Disks

• A shorter wavelength light can read and write
  bytes in greater densities than can be done by a
  longer wavelength of the laser.
• This is one reason that DVDs density is greater
  than that of CD.
• The manufacture of blue-violet lasers can now be
  done economically, bringing about the next
  generation of laser disks.
• Two incompatible formats, HD-CD and Blu-Ray, are
  competing for market dominance.

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7.7 Optical Disks

• Blu-Ray was developed by a consortium of nine
  companies that includes Sony, Samsung, and
  Pioneer.
• Maximum capacity of a single layer Blu-Ray disk
  is 25GB.
• HD-DVD was developed under the auspices of the
  DVD Forum with NEC and Toshiba leading the
  effort.
• Maximum capacity of a single layer HD-DVD is
  15GB.
• Blue-violet laser disks have also been designed
  for use in the data center.
• For long term data storage and retrieval.

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7.8 Magnetic Tape

• First-generation magnetic tape was not much more
  than wide analog recording tape, having
  capacities under 11MB.
• Data was usually written in nine vertical tracks

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7.8 Magnetic Tape

• Todays tapes are digital, and provide multiple
  gigabytes of data storage.
• Two dominant recording methods are serpentine and
  helical scan, which are distinguished by how the
  read-write head passes over the recording medium.
• Serpentine recording is used in digital linear
  tape (DLT) and Quarter inch cartridge (QIC) tape
  systems.
• Digital audio tape (DAT) systems employ helical
  scan recording.

These two recording methods are shown on the next
slide.
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7.8 Magnetic Tape
? Serpentine
Helical Scan ?
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7.8 Magnetic Tape

• Numerous incompatible tape formats emerged over
  the years.
• Sometimes even different models of the same
  manufacturers tape drives were incompatible!
• Finally, in 1997, HP, IBM, and Seagate
  collaboratively invented a best-of-breed tape
  standard.
• They called this new tape format Linear Tape Open
  (LTO) because the specification is openly
  available.

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7.8 Magnetic Tape

• LTO, as the name implies, is a linear digital
  tape format.
• The specification allowed for the refinement of
  the technology through four generations.
• Generation 3 was released in 2004.
• Without compression, the tapes support a transfer
  rate of 80MB per second and each tape can hold up
  to 400GB.
• LTO supports several levels of error correction,
  providing superb reliability.
• Tape has a reputation for being an error-prone
  medium.

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7.9 RAID

• RAID, an acronym for Redundant Array of
  Independent Disks was invented to address
  problems of disk reliability, cost, and
  performance.
• In RAID, data is stored across many disks, with
  extra disks added to the array to provide error
  correction (redundancy).
• The inventors of RAID, David Patterson, Garth
  Gibson, and Randy Katz, provided a RAID taxonomy
  that has persisted for a quarter of a century,
  despite many efforts to redefine it.

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7.9 RAID

• RAID Level 0, also known as drive spanning,
  provides improved performance, but no redundancy.
• Data is written in blocks across the entire array
• The disadvantage of RAID 0 is in its low
  reliability.

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7.9 RAID

• RAID Level 1, also known as disk mirroring,
  provides 100 redundancy, and good performance.
• Two matched sets of disks contain the same data.
• The disadvantage of RAID 1 is cost.

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7.9 RAID

• A RAID Level 2 configuration consists of a set of
  data drives, and a set of Hamming code drives.
• Hamming code drives provide error correction for
  the data drives.
• RAID 2 performance is poor and the cost is
  relatively high.

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7.9 RAID

• RAID Level 3 stripes bits across a set of data
  drives and provides a separate disk for parity.
• Parity is the XOR of the data bits.
• RAID 3 is not suitable for commercial
  applications, but is good for personal systems.

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7.9 RAID

• RAID Level 4 is like adding parity disks to RAID
  0.
• Data is written in blocks across the data disks,
  and a parity block is written to the redundant
  drive.
• RAID 4 would be feasible if all record blocks
  were the same size.

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7.9 RAID

• RAID Level 5 is RAID 4 with distributed parity.
• With distributed parity, some accesses can be
  serviced concurrently, giving good performance
  and high reliability.
• RAID 5 is used in many commercial systems.

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7.9 RAID

• RAID Level 6 carries two levels of error
  protection over striped data Reed-Soloman and
  parity.
• It can tolerate the loss of two disks.
• RAID 6 is write-intensive, but highly
  fault-tolerant.

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7.9 RAID

• Double parity RAID (RAID DP) employs pairs of
  over- lapping parity blocks that provide linearly
  independent parity functions.

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7.9 RAID

• Like RAID 6, RAID DP can tolerate the loss of two
  disks.
• The use of simple parity functions provides RAID
  DP with better performance than RAID 6.
• Of course, because two parity functions are
  involved, RAID DPs performance is somewhat
  degraded from that of RAID 5.
• RAID DP is also known as EVENODD, diagonal parity
  RAID, RAID 5DP, advanced data guarding RAID (RAID
  ADG) and-- erroneously-- RAID 6.

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7.9 RAID

• Large systems consisting of many drive arrays may
  employ various RAID levels, depending on the
  criticality of the data on the drives.
• A disk array that provides program workspace (say
  for file sorting) does not require high fault
  tolerance.
• Critical, high-throughput files can benefit from
  combining RAID 0 with RAID 1, called RAID 10.
• Keep in mind that a higher RAID level does not
  necessarily mean a better RAID level. It all
  depends upon the needs of the applications that
  use the disks.

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7.10 The Future of Data Storage

• Advances in technology have defied all efforts to
  define the ultimate upper limit for magnetic disk
  storage.
• In the 1970s, the upper limit was thought to be
  around 2Mb/in2.
• Todays disks commonly support 20Gb/in2.
• Improvements have occurred in several different
  technologies including
• Materials science
• Magneto-optical recording heads.
• Error correcting codes.

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7.10 The Future of Data Storage

• As data densities increase, bit cells consist of
  proportionately fewer magnetic grains.
• There is a point at which there are too few
  grains to hold a value, and a 1 might
  spontaneously change to a 0, or vice versa.
• This point is called the superparamagnetic limit.
• In 2006, the superparamagnetic limit is thought
  to lie between 150Gb/in2 and 200Gb/in2 .
• Even if this limit is wrong by a few orders of
  magnitude, the greatest gains in magnetic storage
  have probably already been realized.

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7.10 The Future of Data Storage

• Future exponential gains in data storage most
  likely will occur through the use of totally new
  technologies.
• Research into finding suitable replacements for
  magnetic disks is taking place on several fronts.
• Some of the more interesting technologies
  include
• Biological materials
• Holographic systems and
• Micro-electro-mechanical devices.

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7.10 The Future of Data Storage

• Present day biological data storage systems
  combine organic compounds such as proteins or
  oils with inorganic (magentizable) substances.
• Early prototypes have encouraged the expectation
  that densities of 1Tb/in2 are attainable.
• Of course, the ultimate biological data storage
  medium is DNA.
• Trillions of messages can be stored in a tiny
  strand of DNA.
• Practical DNA-based data storage is most likely
  decades away.

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Chapter 7 Conclusion

• I/O systems are critical to the overall
  performance of a computer system.
• Amdahls Law quantifies this assertion.
• I/O control methods include programmed I/O,
  interrupt-based I/O, DMA, and channel I/O.
• Buses require control lines, a clock, and data
  lines. Timing diagrams specify operational
  details.
• Magnetic disk is the principal form of durable
  storage.

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Chapter 7 Conclusion

• Disk performance metrics include seek time,
  rotational delay, and reliability estimates.
• Other external data storages are Optical disks,
  Magnetic tapes,and RAID systems.
• Any one of several new technologies including
  biological, holographic, or mechanical may
  someday replace magnetic disks.
• The hardest part of data storage may be end up be
  in locating the data after its stored.

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